The present invention relates to a lighting panel for a display such as liquid crystal display (LCD) for illuminating the display from the underside thereof as a back light.
FIG. 10 is a perspective view showing a conventional lighting device, and FIG. 11 is a side view of the device.
A lighting panel 102 made of a transparent plastic has a rectangular shape in plan view and has a discharge surface 102a, a reflection surface 102b and an incident surface 102c. Three edge lights 101 of an LED are mounted on a supporting plate 101b corresponding to the incident surface 102c. On the reflection surface 102b, a plurality of prisms 102b1 are formed as reflector members. Each prism 102b1 comprise a first inclination side 102b11 opposite to the incident surface 102c and a second inclination side 102b12 opposite to the first side.
The light emitted from the edge light 101 enters the lighting panel 102. The light in the lighting panel 102 is reflected by the prisms 102b and discharged from the discharge surface 102a. The discharged light enters a prism sheet 103 and discharges from the prism sheet to be applied to an LCD 107.
Light discharged from the underside of the lighting panel 102 is reflected by a reflector sheet 106 to be returned to the lighting panel 102.
Referring to FIG. 12, light emitted from the edge light 101 enters the lighting panel 102 at an incident angle θi, and the light proceeds in the panel 102 in a direction of θ in accordance with the Snell's law. There are following relationships between angle θi and angle θ.θ=sin−1((1/n)sin θi)  (1) where n is the refractive index of the panel, and the refractive index of air is 1.
For example, if the refraction index n of the lighting panel 102 is n=1.58 and θi=90°,θ=sin−1(1/1.58)=39.3°
Therefore, the critical angle θc isθc=39.3°
The incident light is reflected by the discharge surface 102a at a reflection angle θ1 which is larger than 50°. The reflected light strikes the second inclination side 102b12 having an inclination angle α at angle θ2=θ1−α, where α is between 1 degree and several degrees. The light is reflected by the second side 102b12 at the angle of θ2 and strikes the discharge side 102a at an incident angle θ3, θ3=θ2−α=θ1−2α. The light is reflected by the surface 102a at the reflection angle θ3, and strikes the second side 102b12 at an incident angle θ4=θ3−α=θ1−3α.
Thus, the incident angle θ1 reduces α by α at every incidence. Namely in the case of N incidence times, the incident angle θN isθN=θ1−Nα  (2) 
When the incident angle θN becomes smaller than the critical angle θc as followsθN=θ1−Nα<θc  (3) The light discharges from the discharge surface 102a at an incident angle θ5. Consequently, the number of discharge light from an area near the incident surface 102c is small.
Furthermore, there is following described troubles in the conventional lighting panel.
Referring to FIG. 13, there appears bright lines 104 in a range S1 near the incident surface 102c. In a range S2, lights are uniformly and thickly discharged, hence there is no bright lines. The reason for the generating of the bright lines will be described hereinafter.
Referring to FIG. 14, lights from the LED 101 striking a corner 102d of the incident surface 102c enter in the lighting panel 102 from the corner 102d. The lights are diffused in the lighting panel. Light s21 of an incident angle θb smaller than the critical angle θc transmits the second inclination side 102b12 and is reflected by the reflector sheet 106 and enters again in the lighting panel 102. The light s21 transmits the lighting panel 102 and the prism sheet 103. Light s22 of an incident angle θb larger than the critical angle θc is reflected by the second inclination side 102b12 and transmits the lighting panel 102 and prism sheet 103.
The number of incidences is determined before discharge of light by the difference between the incident angle θb and the critical angle θc as described above. The number increases with the difference.
Referring to FIG. 15, φ1, φ2, φ3, φ4 are from the corner 102d. Each of incident rays of light angles θd1, θd2, θd3 and θd4 of rays φ1, φ2, φ3 and φ4 is smaller than the inclination angle α of the second inclination side 102b12. The relationship between the angles θd1, θd2, θd3, θd4 and the critical angle θc are as follows.θd1=1.5α+θc θd2=2.5α+θc θd3=3.5α+θc θd4=4.5α+θc  (4) 
As shown in FIG. 15, the incident angles of the first incidence to the second inclination side 102b12 are as follows.θd1−α, θd2−α, θd3−α, θd4−α
These angles are larger than the critical angle θc as understood from the formula (4). Consequently, all of the rays are reflected.
As the second incidence to the discharge surface 102a, incident angles are as follows.θd1−2α<θc, θd2−2α>θc, θd3−2α>θc, θd4−α>θc 
Only the rays φ1, the incident angle of which is smaller than the critical angle θc, discharges from the discharge surface 102a at a width b1 of rays.
The reflection and discharge at third and fourth incidences are as shown in FIG. 15.
Widths of the rays increase with length of each ray in the lighting panel 102 as b1<b2<b3<b4, as shown in FIG. 15. Thus, the bright lines 104 generate as shown in FIG. 13. Such bright lines decrease the lighting effect for the LCD.